In semiconductor manufacturing plants and other installations requiring extremely high purity gas it is conventional to use electropolished stainless steel (SUS 316L) pipes. Non-welded fitting type or union joints of special construction are frequently used to join the pipes. However, non-welded joints are subject to gas leakage and piping using non-welded joints is extremely expensive. Therefore, in high purity gas piping systems it is conventional to make pipe connections by automatic butt welding by so-called TIG welding, or by automatic welding processes using special welding joints or collars.
In systems conveying very high purity gases, the mechanical strength of welding joints is a major concern. In addition, the welds should not serve as a source of contamination or contaminating particles. Furthermore, the inner surface of the weld joint should be smooth with minimum surface irregularity so as to enable easy formation of a corrosion resistant passive film on the inner surface of the joint. Japanese patent application no. 3-216158 discloses a welding method which nearly completely prevents the generation of particles at the weld joint. In that method, a back shielding inert gas such as argon is discharged into one of the two pipes being welded while an end of the other pipe is kept open. By adjusting the flow rate of the back shielding gas, the internal pressure of the pipes is raised so that any particles formed during the welding process are directed outwardly of the weld site rather than into the pipes. In a conventional welding process in which no back shielding gas is used, several thousand particles on the order of 0.1.mu. or more in size may be released into the piping during welding but when a back shielding gas is used the number of particles released may be reduced to less than a few score.
In the welding process using back shielding high purity gas the internal pressure in the pipes being welded must be held to a specified value by allowing the back shielding gas to flow into the pipes at a specified rate. If the internal pressure cannot be maintained then the number of particles entering the pipes increases greatly. Thus, the use of back shielding gas may be of limited value in certain situations such as, for example, where the welding is carried out near a semiconductor manufacturing apparatus so that the increase in pressure in the pipes by the back shielding gas is restricted, or where the discharge of a large volume of back shielding gas is restricted. Under these conditions it is difficult to sufficiently reduce the number of particles entering the pipes.
By conducting dust generating tests during arc welding of pipes, we have determined that in the case of butt welding a large number of particles enter the pipes within the first 1 to 2 seconds after the welding arc is initiated. On the order of several thousand particles larger than 1 .mu.m may enter the pipes during this interval. Even when the pipe butt sections and the welding head inside the welder are kept clean and the arc and back shielding gases are highly filtered to remove particles, several hundred particles may enter the pipes during the first two seconds after the arc is started.
The dust generating tests indicate that the particles are generated as the result of two phenomena which occur substantially simultaneously. First, at the start of arcing the plasma flow associated with generation of the arc blows a large number of particles up into the welding head. Secondly, solidification shrinkage of the surface of molten metal generated at the start of arcing pulls the arc-starting section of the joint groove thus producing a clearance on the side opposite to the arc-starting section.